1. Field of the Invention
The invention relates to the radar field and more particular to a flat antenna adapted more particularly to ground surveillance radar.
2. Description of the Prior Art
From French patent N.degree.80 16620 in particular, a flat lattice antenna structure is known, having a three-plate line and formed of a source lattice. The antenna includes an energy distribution line placed between two insulating plates, each of these insulating plates being covered with a conducting layer. On the active face of the antenna, the sources are etched, in the copper, the other face of the antenna having a continuous copper layer. The two insulating plates are bonded and the assembly forms the radiating structure.
The use of this type of antenna, more particularly for a ground surveillance radar which might be transportable, requires a certain number of problems to be solved. In fact, the material forming the insulator is a material of the non woven glass-teflon type, of the type for example sold under the trademark DUROID 5880 by the firm RODGERS, chosen for minimizing the losses particularly at high frequency, namely in band X between 9 and 10 GHz. This material is a very good electrical insulator, but it has medicore mechanical characteristics.
In particular, this material has very different expansion coefficients in the three directions. For a flat antenna having dimensions of 900 mm.times.450 mm, the corresponding extension may reach 3 mm. Now, the radiating free of the antenna must be perfectly flat, the tolerance being less than 1 mm. In addition to these problems of expansion, the rigidity of the antenna must of course be ensured even apart from temperature variations.
For a good mechanical strength, an immediate solution consists in placing this antenna on a support, for example a steel support, or a mechanical structure of the mechanical "honeycomb" material type. Calculations show that in order to absorb the above mentioned effects, the support should be very rigid. Furthermore, the radiating face of the antenna must also be protected by a radome which is transparent to the radiation. This radome further adds to the weight of the structure and makes the assembly very difficult to transport.